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A study conducted by Bruno Scarpa at the University of Pavia that investigates the relationship between cervical mucus characteristics on the day of intercourse and the probability of conception. The study analyzes data from a large prospective Italian study using the Ovulation Method to provide estimates of the probabilities of conception according to type of cervical mucus. The document also explores statistical methods for selection of predictors of day-specific conception probabilities when data on ovulation timing are not available.
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University of Pavia
Cervical mucus quality andthe probability of conception:results from an Italian study
Summary To discuss statistical evidence of the effect of cervical mucuson the probability of conception To provide estimates of the probabilities of conceptionaccording to type of cervical mucus classified by the womanon the day of intercourse To explore statistical methods for selection of predictors ofday-specific conception probabilities when data on ovulationtiming are not available
Primary purpose of the study
Target population - sample
^ Prospective cohort study approved by the Institutional Review Board ofFondazione Lanza (Padua, Italy) ^ Co-ordination of the study was made in the Department of Statistical Sciences ofthe University of Padua (prof. Bernardo Colombo) ^ The study entry criteria for the subjects were:^
the woman was experienced in the use of the
Billings Ovulation Method
^ the woman was married or in a stable relationship; ^ the woman was between 18th and 40th birthday at admission; ^ the woman had at least one menses after cessation of breastfeeding or after delivery(or miscarriage); ^ the woman was not taking hormonal medication or drugs affecting fertility ^ Neither partner could be permanently infertile and both had to be free from any illnessthat might cause sub-fertility. ^ It was strictly required that couples did not have the habit of mixing unprotected withprotected intercourse. Women were excluded if any one of these criteria was not fulfilled.
Billings method
^ mucus peak
: “The last day of the cycle during which at least one characteristics of high fertility in mucus type has been observed or felt, considering characteristicsof high fertility a wet sensation and/or the observation of slippery, transparent,liquid or watery mucus, or of blood trails.Moreover, this day must be preceded by an adequate growth in sensation andappearance of mucus characteristics, which should also show afterwards a clearchange to the less fertile” Ovulation is expected within two days after the peak: this can then be used as areference for the determination of the end of the fertile phase. When in a cycle no peak is detected, it is not possible to judge if and whenovulation did occur and, therefore, to identify a
postovulatory infertile phase
^ Co-operation of 4 Italian centres providing services on the Billings ovulationmethod
Mrs. Elena Giacchi, MD
Centro Studi e Ricerche Regolazione Naturale della Fertilità
Sr. Erika Bucher
Associazione Metodo Billings Emilia-Romagna, AMBER
Mrs. Lorella Miretti, RN
Centro Piemontese Metodo Billings, CEPIMB
Mrs. Medua Boioni
Centro Lombardo Metodo Billings-CLOMB
^ During the years 1993-97 were recruited 193 women ^ A few subjects had kept long series of past own observations. So long as itsatisfied all the criteria of the programmed protocol, also this little piece ofinformation was utilized in the construction of the data base.
Target population - sample
Target population - sample
Mucus classification
Sensation
Appearance
0
No information
No information
1
No sensation or dry sensation
No mucus nor anyinsubstantial loss
2
No longer dry sensation
No substantial discharge, nor any
noticeable mucus
3
Damp sensation
Thick, creamy, whitish, yellowish
sticky, stringy mucus
4
Wet, liquid sensation
—
5
Wet-slippery sensation
Clear, stringy (or stretchy), fluid,watery mucus, blood trails
*If during a day there are different observations of the mucus symptom, the codingis determined by the most fertile type
Wet sensation 4
Descriptive statistics
Code
Mean
Median
Interquartile Range
DeviationStandard
Mucus type and day of the cycle
III. Mucus and peak day as ovulation marker
Pf,j
is the probability of fertilization in cycle
j^ of a fertilizable ovule.
^ M
=(M0ij
, M2, M3ijij
, M4,)ijij
T^ is the vector of dummy variables which indicates the presence of
different mucus codes (
0,2,3,4,
and^1
is the reference code) for a specific day
i^ within a cycle
j.
^ We assumed for the fertilization probability
a logit relation
^
is the effect on the probability of fertilization depending on the specific position of day
i
^
( h^ = 0, 2, 3, 4
) is the effect on the probability of fertilization in the logit
scale due to the presence of mucus of code
h.
^ The parameters estimation can be obtained through standard maximum likelihoodprocedures.
(^ )^
ij
β δ +
δ^ i
(^
)
(^
)
∏^
−
i
X ij i
jf j
ij
βδ
,
(^
) (^43) ,,,^ β β β β (^20) β^ =
α^ i
III. Estimates δ 8 −
δ^1
δ^7 −
δ^2
δ^6 −
δ^3
δ^5 − δ^4 −
β^0
δ^3 −
β^2
δ^2 −
β^3
δ^1 −
β^4
Parameter^ δ^0
Estimate
Lower – Upper90% Interval
Parameter
Estimate
Lower – Upper90% Interval
-26.
(-∞, -4.975)
-2.^
(-4.625,-1.228)
-4.^
(-6.604, -3.434)
-2.^
(-4.676, -1.426)
-3.^
(-5.077, -2.574)
-4.^
(-5.568, -2.790)
-4.^
(-5.808, -2.571)
k^
(0.378, 1)
-1.^
(-3.318, -0.343)
(-1.014, 3.571)
-1.^
(-3.537, 0.927)
(0.468, 3.223)
-2.^
(-4.496, -0.731)
(0.892, 3.080)
-1.^
(-3.729, 0.507)
(0.228, 2.976)
0.^
(-2.797, +
∞)
{^ }^ βexp h
^ No information to distinguish female factors, suchas cycle viability, from male factors ^ No way to reliably interpret
w^ &
p as separate k^
biological parameters from the data Difficult to separately estimate
w^ and
max
pk k^
on occurrence of multiple intercourse acts Bottom line
: over-parameterized & unstable
Problems with Schwartz Model model, even without predictors & heterogeneity
Generalization of Barrett and Marshall model
An alternative model: Dunson and Stanford (2005)
Conception in a cycle with^ X pattern of intercourse ij^ and
U expl. variables ij^
